The present invention relates to a method for eliminating undesirable charged low-mass particles from the measuring cell of an ion cyclotron resonance spectrometer, the measuring cell being arranged within a homogeneous magnetic field and defined by trapping electrodes which are provided in perpendicular arrangement relative to the direction of the magnetic field and maintained at a trapping potential generating a trapping field for the charged particles, and by transmitting electrodes provided in parallel arrangement relative to the direction of the magnetic field and supplied with an rf voltage serving to excite the cyclotron movement, the said method including the step of exciting the charged particles to be eliminated by modulation of the trapping field, to perform trapping oscillations in the direction of the homogenous magnetic field so that they overcome the trapping potentials and reach the trapping electrodes for being eliminated via the trapping electrodes.
The elimination of low-mass charged particles from the measuring cells of ion cyclotron resonance or, shortly, ICR spectrometers is of great importance in quite a number of experiments. For example, the elimination of He.sup.+ ions is very important in gas chromatography experiments with a view to improving the dynamic range of the instrument. A great number of He.sup.+ ions originating from the carrier gas lead to considerable space-charge effects which impair the effectiveness of the ion trap if no measures are taken to remove the He.sup.+ ions before signal detection occurs. Similarly, when working with negative ions, trapped electrons may impair the experiments in an undesirable manner. Consequently, it is an important aspect of high-power ICR spectrometers that they provide the possibility of eliminating such low-mass particles.
However, eliminating these particles cannot easily be effected with the aid of the usual double-resonance technique because the cyclotron resonance frequencies of such light-weight particles are very high. Given a strength of the homogenous magnetic field of 4.7 Tesla, the cyclotron resonance frequency of electrons is in the range of approximately 140 GHz. Such a high frequency is far beyond the frequency range of the electronic equipment of usual ICR spectrometers. So, it has been known for example from "messtechnik" 78 (6, 1970), 109 to 195, to eliminate such light-weight particles by utilization of the harmonic oscillations performed by these particles along the magnetic field line, i.e. the so-called trapping oscillation. From a paper by McIver et al published in "Lecture notes in chemistry", (1982), pages 164 to 182, it has also been known to connect an oscillator to the trapping electrodes which provides the possibility of exciting electrons to perform trapping oscillations in the direction of the magnetic field until they get into contact with the trapping electrode and are, thus, eliminated. However, it is a drawback of this method that it requires the use of an additional, tunable transmitter since the resonance frequency of the trapping oscillations depends not only on the charge/mass ratio of the particles to be eliminated, but also on the potential applied to the trapping electrodes.